1. Field of the Invention
The present invention relates to a lithography system using a charged-particle beam for fabricating semiconductor integrated circuits, especially LSIs. More particularly, the present invention relates to a lithography system, in which a charged-particle beam is deflected to pass through a selected block of a stencil pattern on a stencil mask on its way toward an objective and forms an image of the selected stencil pattern thereon.
2. Description of the Related Art
A lithography system using a charged-particle beam, particularly, a system using an electron beam has been widely used for either fabricating a mask or reticle or directly patterning a resist layer on a wafer. Generally, an electron beam lithography system utilizes a rectangularly-shaped electron beam which is produced by making the electron beam pass through two rectangular apertures, the rectangular beam dimensions being adjusted by changing overlapping relation between the two apertures. The rectangular-shaped electron beam is made to move on a wafer under the control of a pattern generating unit. By connecting each exposed rectangular area, the required pattern can be drawn. The electron beam lithography system has established an important position due to its high precision and high speed in patterning.
When the pattern to be drawn on the wafer becomes more intricate and minute, the number of exposure shots increases sharply, which reduces throughput of the lithography system. In order to improve the throughput in exposing superfine patterns, a stencil mask method has been proposed. Generally, an LSI pattern includes a lot of repetition of a basic unit pattern, therefore, if each basic unit pattern can be exposed in one exposure shot, the throughput of exposure can be improved remarkably. The stencil mask comprises a plurality of blocks of basic unit stencil pattern. When an electron beam is made to pass through a selected one of stencil pattern blocks during its progress toward the wafer, an outline of the electron beam is shaped to the selected stencil pattern and the reduced stencil pattern is transcribed on the wafer by one exposure shot. If the exposure shot is repeated plurality of times moving the electron beam on the wafer, the necessary pattern can be exposed comparatively easily in a short time.
It is preferable that many blocks of stencil pattern are formed on the stencil mask and can be accessed by electrically deflecting the electron beam. For example, if a single block of stencil pattern is allowed to occupy a 500 .mu.m square on the stencil mask and an electrically accessible number of stencil pattern blocks is assumed to be one hundred, the exposure unit is required that the electron beam can be deflected over an area of about 6 mm square on the stencil mask.
Further, the electron beam is required to satisfy the following conditions. First, the electron beam can be deflected onto the specific stencil pattern block as described above and the incident beam onto the selected stencil pattern is vertical thereto. Second, the electron beam needs to create a sharp focused image on the stencil mask. Third, the electron beam having displacement from the center axis should be deflected back to the center axis position. Fourth, a sharp focused image of the stencil pattern is to be formed on the wafer.
Two pairs of either electrostatic or electromagnetic deflectors, each pair disposed in opposed relationship with regard to the stencil mask, deflect the electron beam onto the selected stencil pattern from the axis and deflect back to the axis, satisfying the above first and third conditions. However, due to the large beam displacement from the center axis, beam aberration problems such as astigmatism and field curvature can not be avoided. Both problems cause a blurred image on either the stencil mask or the wafer. In order to solve the problems, deflection and focus system of the exposure unit should be separated into two, one on the upstream side of the stencil mask and the other on the downstream side thereof. The above problems should be solved for both upstream and downstream sides of the stencil mask.